1,779 research outputs found
Optical alignment and polarization conversion of neutral exciton spin in individual InAs/GaAs quantum dots
We investigate exciton spin memory in individual InAs/GaAs self-assembled
quantum dots via optical alignment and conversion of exciton polarization in a
magnetic field. Quasiresonant phonon-assisted excitation is successfully
employed to define the initial spin polarization of neutral excitons. The
conservation of the linear polarization generated along the bright exciton
eigenaxes of up to 90% and the conversion from circular- to linear polarization
of up to 47% both demonstrate a very long spin relaxation time with respect to
the radiative lifetime. Results are quantitatively compared with a model of
pseudo-spin 1/2 including heavy-to-light hole mixing.Comment: 5 pages, 3 figure
Remarks on hard Lefschetz conjectures on Chow groups
We propose two conjectures of Hard Lefschetz type on Chow groups and prove
them for some special cases. For abelian varieties, we shall show they are
equivalent to well-known conjectures of Beauville and Murre.Comment: to appear in Sciences in China, Ser. A Mathematic
ISM gas studies towards the TeV PWN HESS J1825-137 and northern region
HESS J1825-137 is a pulsar wind nebula (PWN) whose TeV emission extends
across ~1 deg. Its large asymmetric shape indicates that its progenitor
supernova interacted with a molecular cloud located in the north of the PWN as
detected by previous CO Galactic survey (e.g Lemiere, Terrier &
Djannati-Ata\"i 2006). Here we provide a detailed picture of the ISM towards
the region north of HESS J1825-137, with the analysis of the dense molecular
gas from our 7mm and 12mm Mopra survey and the more diffuse molecular gas from
the Nanten CO(1-0) and GRS CO(1-0) surveys. Our focus is the possible
association between HESS J1825-137 and the unidentified TeV source to the
north, HESS J1826-130. We report several dense molecular regions whose
kinematic distance matched the dispersion measured distance of the pulsar.
Among them, the dense molecular gas located at (RA,
Dec)=(18.421h,-13.282) shows enhanced turbulence and we suggest that
the velocity structure in this region may be explained by a cloud-cloud
collision scenario. Furthermore, the presence of a H rim may be the
first evidence of the progenitor SNR of the pulsar PSR J1826-1334 as the
distance between the H rim and the TeV source matched with the
predicted SNR radius R~120 pc. From our ISM study, we identify a
few plausible origins of the HESS J1826-130 emission, including the progenitor
SNR of PSR J1826-1334 and the PWN G018.5-0.4 powered by PSR J1826-1256. A
deeper TeV study however, is required to fully identify the origin of this
mysterious TeV source.Comment: 19 figures, 27 pages, accepted by MNRA
SAR image classification with non-stationary multinomial logistic mixture of amplitude and texture densities
We combine both amplitude and texture statistics of the Synthetic Aperture Radar (SAR) images using Products of Experts (PoE) approach for classification purpose. We use Nakagami density to model the class amplitudes. To model the textures of the classes, we exploit a non-Gaussian Markov Random Field (MRF) texture model with t-distributed regression error. Non-stationary Multinomial Logistic (MnL) latent class label model is used as a mixture density to obtain spatially smooth class segments. We perform the Classification Expectation-Maximization (CEM) algorithm to estimate the class parameters and classify the pixels. We obtained some classification results of water, land and urban areas in both supervised and semi-supervised cases on TerraSAR-X data
Efficient dynamical nuclear polarization in quantum dots: Temperature dependence
We investigate in micro-photoluminescence experiments the dynamical nuclear
polarization in individual InGaAs quantum dots. Experiments carried out in an
applied magnetic field of 2T show that the nuclear polarization achieved
through the optical pumping of electron spins is increasing with the sample
temperature between 2K and 55K, reaching a maximum of about 50%. Analysing the
dependence of the Overhauser shift on the spin polarization of the optically
injected electron as a function of temperature enables us to identify the main
reasons for this increase.Comment: 5 pages, 3 figure
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